When operating pod units in arctic seas, very stiff propeller blades are required. This means that also a very stiff shaft is required in order to avoid that the motor inside the pod is damaged in case the propeller hits ice or some other hard object, e.g. is grounded, whereby the shaft will be exposed to a bending force.
Another situation that could cause high bending forces (and risk of damaging the motor) would be when high shock loads are encountered, e.g. due to an explosion of a mine. As a consequence this might be required as a dimensioning criterion for certain projects, e.g. for navy vessels.
To avoid a detrimental bending of the shaft, it would be obvious to use a very stiff shaft. However, this means that that shaft would have to have a very large diameter, which is costly, increases the weight of the pod unit and further requires space that not always is available.
WO 2010/108544 A2 discloses a bearing assembly for an electrical motor, comprising a shaft, a housing, and a main bearing between the shaft and the housing, wherein the shaft is surrounded by a rigid sleeve for functioning as an auxiliary bearing in case of a breakdown of the main bearing and as a grease sealing under normal operation. The clearance between the sleeve and the shaft is smaller than the air-gap between the stator and the rotor of the motor. The clearance is at most 0.6 mm but may be at most 0.3, 0.2, 0.1 or 0.05 mm, while the air-gap between the stator and the rotor typically is 1.2-1.5 mm. With reference to the interior of the pod housing, the sleeve is shown positioned just inside the main bearing. There is no indication that such an arrangement would make it possible to reduce the required diameter of the shaft, i.e. to use a weaker shaft than else would be possible.